The present invention relates to a transparent rear projection screen having a lense system for paralleling light arriving from behind and, on the front side of the screen, rectilinear lenses extending in the vertical direction in the position of use of the screen, the sides of the lenses having a small angle of inclination with respect to the normal to the screen and being, at least along a part of their width, totally reflecting to light arriving from behind, and comprising further lenses arranged between the reflecting lenses.
Screens of the kind referred to above are known from European Patent Application No. 82102985.7, publication No. 0063317. According to said prior art the reflecting lenses are so constructed that the light beams which extend parallel with the normal to the screen and impinge the inner surfaces of the totally reflecting lense side parts are reflected out through the tops of the lenses concerned. The tops of the lenses are convexly rounded and form sharp corners with respect to the sides of the lenses concerned, viz. in such a way that beams which from the inner side impinge the side parts of the tops of the lenses, and which extend parallel with the normal to the screen, are not totally reflected but on the contrary pass out through the top of the lense while being refracted. The totally reflecting lense side parts result in that the light emitted from the tops of the lenses is refracted more in the lateral direction than otherwise would be the case, because the totally relfected light, before passing out through the tops of the lenses, has been diverged an angle corresponding to twice the angle which the totally reflecting parts form together with the normal to the screen. Between the lenses referred to above, trough-shaped grooves are provided the bottoms of which constitute convex or concave further lenses. The further lenses will diverge light extending parallel with the normal to the screen and impinging the inner surfaces of the further lenses. However, the spreading which may be achieved by means of such further lenses is rather limited and this is the reason why the lenses having totally reflecting side parts referred to above are used which, as previously explained, have the effect that the spreading of the light emitted through the tops of these lenses will be greater than the spreading of light emitted from lenses impinged solely by light arriving parallel with the normal to the screen. Accordingly, due to the totally reflecting lense side parts it is achieved that the image projected may be seen at wider side viewing angles than the case otherwise would be.
The screens referred to above are used in connection with video projectors, for micro film readers and as projection screens for computers.
It is the object of the present invention to provide a screen as stated above which in particular is suitable for projecting colour television images. It is known to produce coloured projected television images by directing three projectors, each projecting a black-white television image against the rear side of a transparent projection screen. The images from each of the three projectors are coloured, e.g. red, green and blue, respectively, by positioning a correspondingly coloured filter in front of the picture tube of each of the projectors. The three projectors are arranged side by side and are directed in such a way that the images emitted from the tubes cover each other as far as possible when the images are watched from the front side of the screen. Due to the fact that the three projectors cannot be arranged with their optical axes coaxially, the projectors are arranged side by side but such arrangement, on the other hand means that their optical axes will form angles with each other. Normally it is estimated that the projectors cannot be arranged closer adjacent each other than corresponding to an angle of 7.degree. between the optical axes of the projectors. However, the development in direction towards projectors having increasing light intensity may result in that the angles referred to may increase even up to 12.degree..
If e.g. the green image is projected perpendicular upon the rear side of the screen, the blue and the red images will be projected upon the rear side of the screen at an inclination of e.g. said 7.degree. with respect to the normal to the screen. However, the result will be that a person, which watches the screen obliquely from in front, will see an image which is either blue or red dominated depending upon whether the person concerned is positioned closer to the optical axis of the projector emitting the blue or to the axis of the projector emitting the red image. Such colour distortion will in the present specification be termed "colour shading".